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Pentaquark searches at ZEUSPowerPoint Presentation

Pentaquark searches at ZEUS

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R. Yoshida (Argonne)

Pentaquark searches at ZEUS- Θ KSp : Many fixed target experiments report Θ+ K+n (and KSp) at ~ 1530 MeV
- Ξ3/2 Ξπ : NA49 reports exotic Ξ3/2 at ~1860 MeV
- ΘC D*p : H1 reports ΘC D*p at 3099 MeV

Θ KSp

- Θ+ K+n is a manifestly exotic state (uudds). Observed mass ~1530 MeV.
- Θ KSp is not manifestly exotic. Can be interpreted as a Σ* (uudds).
- KSp spectrum has many reported “Σ bumps” (observed by experiments at low significance). 1480 MeV (near threshold), 1560 MeV, 1580 MeV,…
- A “window” exists 15001550 MeV where no states are reported.
- A narrow state at ~1530 MeV in KSP would be consistent with the exotic Θ+.

Θ KSp

The search is made in Deep Inelastic Scattering

sample: ~121 pb-1, the entire HERA I sample.

electron (triggers event without bias)

Beamhole

Proton

Initial baryon

number

escapes

Fragmentation

Θ KSp

870k Ks reconstructed

(anti)Proton candidates

from dE/dx in the

central tracking

chamber (CTD).

(produce Θ in

the same way

as baryons in

fragmentation)

Θ KSpSpectrum in bins of Q2.

At Q2 ≈10GeV2 or larger,

Θ/event≈const

Θ/bgd≈const

As Q2 0

Θ/event

Θ/bgd

Θ KSp

1560

1480

1660

M = 1521.5 ± 1.5 MeV

Γ = 8 ± 4 MeV

1580

1620

1670

Monte Carlo normalized

here

Monte Carlo has no

knowledge of “Σ bumps”

Particle and antiparticle

(Results same for Q2 > 15-30 GeV2)

Θ KSp

Determine cross-section using MC (Σ+ forced to be Θ+)

Θ KSp

Cross-section ratio to the Λ (1116) determined using Λpπ mode.

Λ(1520) K+p has very different kinematic selections due to the use of dE/dx for both

K and p.

5%

Θ KSp

- A state at 1521.5 ± 1.5 MeV and width Γ = 8 ± 4 MeV observed in 121 pb-1 of DIS sample in the Ksp spectrum at Q2>20 GeV2.
- The significance is 3.9-4.6σ (exact number cannot be determined due to the unknown background shape.)
- Interpretation:
- Same state as Θ+ pentaquark observed elsewhere. Then:
- First observation in fragmentation
- First observation of the anti-pentaquark

- A so-far-unobserved Σ*.
- Structure due to some interference. (Reflections from known states were checked and ruled out)

- Same state as Θ+ pentaquark observed elsewhere. Then:

Ξ3/2 Ξπ

NA49 has reported the observation of doubly charged

decuplet partner of the Θ, the Ξ3/2 as well as the Ξ3/2 .

o

ZEUS has searched for this state in the same 121 pb-1 DIS sample as for the Θ search.

First reconstruct

Λ using dE/dx to ID

protons.

Ξ3/2 Ξπ

Combine Λ with π to reconstruct Ξ

Ξ3/2 Ξπ

No peaks observed in any of the combinations.

Ξ3/2 Ξπ

- No Ξ3/2 state was observed.
- The number of Ξ(1530) observed in this study is about the same as for the NA49 study.
- The ratio R=N(Ξ3/2 )/N(Ξ(1530) ) around 1860 MeV is <0.29 at 95% C.L.
- Since this is a search in the fragmentation region—does not necessarily contradict the NA 49 result.

ΘC D*p

- H1 collaboration has reported an observation of a narrow state ΘC D*p at 3099 MeV in DIS.
- The signal corresponds to ~1% of the observed number of D*s (51±11 ΘCs observed)
- H1 also reports a compatible state observed in photoproduction (Q2≈0).

ΘC D*p

ZEUS has made a search in 126 pb-1 of DIS and photoproduction data.

Both K2π and K4π decay modes of D* were used. (H1 uses K2π only).

Altogether, 62600 D*s are found. 13500 in DIS alone. If 1% of D*s came from ΘC–expect >600 ΘCs.

ΘC D*p

dE/dx was again used to identify protons.

Procedure was somewhat refined—use χ2 probability of a proton hypothesis rather than a simple cut.

ΘC D*p

MC simulation of

ΘCat 1% of observed

D*s.

“1%” signal is ruled out for the whole sample at 9σ

ruled out for DIS alone at 5σ

K2π and K4π

-1.6 < η(D*) < 1.6

PT(D*) > 1.35 GeV (for K2π)

y < 0.95

K2π only

-1.5 < η(D*) < 1.0

For DIS

0.05 > y > 0.7

PT(D*) > 1.5 GeV

For Photoprod.

0.2 > y > 0.8

PT(D*) > 2.0 GeV

..other minor changes

ΘC D*pDefault selection

“H1” selection

Note all cuts are tighter

for the “H1” selection.

ΘC D*p

- No resonance structure observed from >60k D*s.
- “1%” signal is ruled out at 9σ for all and 5σ for DIS only.
- (Also checked Q2>20 GeV2—nothing was found)
- The upper limit of observed D*s originating from ΘCis 0.23% (95% C.L.)
- After acceptance correction: (assuming PT(ΘC) and η(ΘC) same as those of D*): the upper limit of D*s, in the measured kinematic region, originating from ΘCis 0.16% (95% C.L.)
- These results are incompatible with the H1 result.

Summary

- Θ KSp : A narrow state observed at 1520 MeV at Q2>20 GeV2. Maybe the Θ+ pentaquark.
- Ξ3/2 Ξπ : No state is observed. May still be compatible with NA49 result—fragmentation region probed.
- ΘC D*p : No state is observed. A production at 1% of the D* rate is ruled out. Not compatible with H1 result.

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